1. Field of the Invention
The present invention relates to semiconductor devices.
In this specification, a semiconductor device generally means a device which can function by utilizing semiconductor characteristics, and an electrooptic device, a semiconductor circuit, and electronic devices are all semiconductor devices.
2. Description of the Related Art
Attention has been focused on a technique for forming a transistor (also referred to as a thin film transistor (TFT)) using a semiconductor thin film formed over a substrate having an insulating surface. The transistor is applied to a wide range of electronic devices such as an integrated circuit (IC) or an image display device (display device). A silicon-based semiconductor material is widely known as a material for a semiconductor thin film applicable to a transistor. As another material, an oxide semiconductor has been attracting attention.
For example, a transistor whose active layer includes an amorphous oxide containing indium (In), gallium (Ga), and zinc (Zn) is disclosed (see Patent Document 1).